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Scaffolding and docking proteins of the heart, an introduction
Journal of Molecular and Cellular Cardiology 37 (2004) 389–390 www.elsevier.com/locate/yjmcc
Scaffolding and docking proteins of the heart, an introduction Meredith Bond Departement of Physiology, University of Maryland School of Medicine, Baltimore, MD, USA Received, revised and accepted 28 May 2004
The force of cardiac contraction is subject to moment by moment regulation—as a result of changes in preload and afterload, and as a result of neuroendocrine regulation, e.g. by the actions of norepinephrine, angiotensin, cytokines and other circulating factors. These fluctuating changes in the environment of the cardiac myocytes need to be faithfully transduced to cause altered function of a set of proteins within the cell. In order to minimize delays due to diffusion, efficient signaling requires a mechanism of rapid on-and-off switching of the signal within a defined subcellular microdomain. This requirement has been most clearly illustrated for regulation of ion channel function which occurs on a millisecond time scale. This regulation of function occurs primarily by an alteration of protein conformation, as a result of the action of kinases and phosphatases. Examples of ion channel regulation by means of targeting of signaling molecules are discussed in reviews in this series by Bers, Ruehr and Catterall. In particular, the protein complex associated with the ryanodine receptor is a major focus of current interest. Components of signaling pathways need to be in close physical association with each other, i.e. in a macromolecular protein complex, in order for both rapid and transient responses to occur. In other words, proteins needed both for amplifying and dampening signals need to be juxtaposed. Scaffolding proteins represent the structural framework on which multiprotein complexes are tethered. Clustering of signaling molecules in multiprotein complexes eliminates delays that would otherwise occur as a result of diffusion. Conversely, targeting of signaling proteins to scaffolding proteins ensures a high local concentration of signaling molecules. As discussed above, scaffolding proteins commonly target kinases and phosphatases: protein kinase A by A-kinase anchoring proteins (AKAPs [1,2], protein kinase C by receptors for activated C-kinase (RACKs) [3,4], and phosphatases [5,1]),. Scaffolding proteins can also subserve a similar function in other pathways, e.g. clustering of E-mail address: [email protected] (M. Bond). © 2004 Published by Elsevier Ltd. doi:10.1016/j.yjmcc.2004.05.027
caspases by apoptosomes [6], or targeting of phosphorylated substrates by 14–3–3 proteins [7] The scaffolding molecules may themselves be directly involved in signaling, in addition to having a scaffolding function (e.g. Ga, b and c subunits [4]). Finally, localization of signaling molecules can be facilitated by the membrane environment itself, as illustrated by the importance of localization of b2-adrenergic signaling components to membrane micro-domains, such as caveolae [8]. As proteomic and mass spectrometry approaches become more readily available as tools to identify members of macromolecular protein complexes which subserve signaling functions, we will identify many new proteins of unknown function. In future studies, we will therefore need to increasingly focus our efforts on understanding the role of these novel proteins in regulating the signaling pathways with which they are associated. In other words, in the future, we will find ourselves working more and more in a discovery mode. Whereas we are able to identify known and now, novel proteins which are members of macromolecular protein complexes targeted by scaffolding proteins, we need to also focus our efforts on the time dimension: the dynamics of exchange of components of protein complexes in intact cells and tissues. Whereas this adds an additional layer of complexity, a multidimensional understanding of the role of scaffolding proteins in both time and space will be essential to a complete understanding of the role of this class of proteins in regulation of cardiac function. Achievement of this goal will require quantitative and structural biology approaches to examine time-dependent changes in binding affinity of components of these complexes. For example, we know that upon dephosphorylation of a kinase substrate, its affinity for 14–3–3 proteins is decreased (Yaffe review). Themes of common binding motifs will also need to be explored—e.g. the role of WD40 repeats and leucine zipper motifs, as discussed in several of the reviews in this series.
390
M. Bond / Journal of Molecular and Cellular Cardiology 37 (2004) 389–390
References
[4]
[1]
[5]
[2]
[3]
Ruehr ML, Russel MA, Bond M. A-kinase anchoring protein targeting of protein kinase A in the heart. J Mol Cell Cardiol 2004;37 in next issue. Hulme JT, Scheuer T, Catterall WA. Regulation of cardiac ion channels by signaling complexes : role of modified leucine zipper motifs. J Mol Cell Cardiol 2004;37 in next issue. Vondriska TM, Pass JM, Ping P. Scaffold proteins and assembly of multiprotein signaling complexes. J Mol Cell Cardiol 2004;37:391–7 next article in this issue.
[6] [7] [8]
Chen S, Bryan D, Spiegelberg, Lin Fan, Dell Edward J, Hamm Heidi E, et al. Interaction of Gbc with RACK1 and other WD40 repeat proteins. J Mol Cell Cardiol 2004;37:399–406 in this issue. Bers DM. Macromolecular complexes regulating cardiac ryanodine receptor function. J Mol Cell Cardiol 2004;37:417–29 in this issue. Czerski L, Nuñez G. Aproptosome formation and caspase activation : is it different in the heart? J Mol Cell Cardiol 2004;37 in next issue. Wilker E, Yaffe MB. 14-3-3 Proteins in human disease-a focus on cancer. J Mol Cell Cardiol 2004;37 in next issue. Steinberg SF. b2-Adrenergic receptor signaling complexes in cardiomyocyte caveolate/lipid rafts. J Mol Cell Cardiol 2004;37:407–15 in this issue.
Scaffolding and docking proteins of the heart, an introduction Meredith Bond Departement of Physiology, University of Maryland School of Medicine, Baltimore, MD, USA Received, revised and accepted 28 May 2004
The force of cardiac contraction is subject to moment by moment regulation—as a result of changes in preload and afterload, and as a result of neuroendocrine regulation, e.g. by the actions of norepinephrine, angiotensin, cytokines and other circulating factors. These fluctuating changes in the environment of the cardiac myocytes need to be faithfully transduced to cause altered function of a set of proteins within the cell. In order to minimize delays due to diffusion, efficient signaling requires a mechanism of rapid on-and-off switching of the signal within a defined subcellular microdomain. This requirement has been most clearly illustrated for regulation of ion channel function which occurs on a millisecond time scale. This regulation of function occurs primarily by an alteration of protein conformation, as a result of the action of kinases and phosphatases. Examples of ion channel regulation by means of targeting of signaling molecules are discussed in reviews in this series by Bers, Ruehr and Catterall. In particular, the protein complex associated with the ryanodine receptor is a major focus of current interest. Components of signaling pathways need to be in close physical association with each other, i.e. in a macromolecular protein complex, in order for both rapid and transient responses to occur. In other words, proteins needed both for amplifying and dampening signals need to be juxtaposed. Scaffolding proteins represent the structural framework on which multiprotein complexes are tethered. Clustering of signaling molecules in multiprotein complexes eliminates delays that would otherwise occur as a result of diffusion. Conversely, targeting of signaling proteins to scaffolding proteins ensures a high local concentration of signaling molecules. As discussed above, scaffolding proteins commonly target kinases and phosphatases: protein kinase A by A-kinase anchoring proteins (AKAPs [1,2], protein kinase C by receptors for activated C-kinase (RACKs) [3,4], and phosphatases [5,1]),. Scaffolding proteins can also subserve a similar function in other pathways, e.g. clustering of E-mail address: [email protected] (M. Bond). © 2004 Published by Elsevier Ltd. doi:10.1016/j.yjmcc.2004.05.027
caspases by apoptosomes [6], or targeting of phosphorylated substrates by 14–3–3 proteins [7] The scaffolding molecules may themselves be directly involved in signaling, in addition to having a scaffolding function (e.g. Ga, b and c subunits [4]). Finally, localization of signaling molecules can be facilitated by the membrane environment itself, as illustrated by the importance of localization of b2-adrenergic signaling components to membrane micro-domains, such as caveolae [8]. As proteomic and mass spectrometry approaches become more readily available as tools to identify members of macromolecular protein complexes which subserve signaling functions, we will identify many new proteins of unknown function. In future studies, we will therefore need to increasingly focus our efforts on understanding the role of these novel proteins in regulating the signaling pathways with which they are associated. In other words, in the future, we will find ourselves working more and more in a discovery mode. Whereas we are able to identify known and now, novel proteins which are members of macromolecular protein complexes targeted by scaffolding proteins, we need to also focus our efforts on the time dimension: the dynamics of exchange of components of protein complexes in intact cells and tissues. Whereas this adds an additional layer of complexity, a multidimensional understanding of the role of scaffolding proteins in both time and space will be essential to a complete understanding of the role of this class of proteins in regulation of cardiac function. Achievement of this goal will require quantitative and structural biology approaches to examine time-dependent changes in binding affinity of components of these complexes. For example, we know that upon dephosphorylation of a kinase substrate, its affinity for 14–3–3 proteins is decreased (Yaffe review). Themes of common binding motifs will also need to be explored—e.g. the role of WD40 repeats and leucine zipper motifs, as discussed in several of the reviews in this series.
390
M. Bond / Journal of Molecular and Cellular Cardiology 37 (2004) 389–390
References
[4]
[1]
[5]
[2]
[3]
Ruehr ML, Russel MA, Bond M. A-kinase anchoring protein targeting of protein kinase A in the heart. J Mol Cell Cardiol 2004;37 in next issue. Hulme JT, Scheuer T, Catterall WA. Regulation of cardiac ion channels by signaling complexes : role of modified leucine zipper motifs. J Mol Cell Cardiol 2004;37 in next issue. Vondriska TM, Pass JM, Ping P. Scaffold proteins and assembly of multiprotein signaling complexes. J Mol Cell Cardiol 2004;37:391–7 next article in this issue.
[6] [7] [8]
Chen S, Bryan D, Spiegelberg, Lin Fan, Dell Edward J, Hamm Heidi E, et al. Interaction of Gbc with RACK1 and other WD40 repeat proteins. J Mol Cell Cardiol 2004;37:399–406 in this issue. Bers DM. Macromolecular complexes regulating cardiac ryanodine receptor function. J Mol Cell Cardiol 2004;37:417–29 in this issue. Czerski L, Nuñez G. Aproptosome formation and caspase activation : is it different in the heart? J Mol Cell Cardiol 2004;37 in next issue. Wilker E, Yaffe MB. 14-3-3 Proteins in human disease-a focus on cancer. J Mol Cell Cardiol 2004;37 in next issue. Steinberg SF. b2-Adrenergic receptor signaling complexes in cardiomyocyte caveolate/lipid rafts. J Mol Cell Cardiol 2004;37:407–15 in this issue.